Heart actually 2 pumps side by side
Right side (ventricle) of the heart pumps blood to the lungs PULMONARY CIRCUIT PUMS - gas exchange
Left side (ventricle) of the heart pumps blood to all body tissue = SYSTEMIC CIRCUIT PUMP functional blood

Describe blood flow to, from & through the heart

Blood returning from the body is relatively oxygen poor & carbon dioxide rich
It enters the RIGHT ATRIUM and passes into the R Ventricle, which pumps it to the lungs via the PULMONARY trunk.
In the lungs the blood off loads carbon dioxide & picks up oxygen.
Freshly oxygenated blood is carried by the Pulmonary veins back to the Left side of the heart (left Atrium) and passes into the Left Ventricle, which pumps it into the Aorta.
From there the blood is transported via smaller systemic arteries to the body tissues, where gases & nutrients are exchanged across the capillary walls.

Length of absolute refractory period (in-excitable period when Na+ channels are still open or inactivated) lasts approximately 250ms, nearly as long as the contraction.
1 - DEPOLARIZATION
2 - PLATEAU PHASE
3 - REPOLARIZATION